1. Field of the Invention
The present invention relates to a cam mechanism, incorporated in a photographing (imaging) lens (lens barrel), for advancing or retracting at least one movable member by a rotation of a cam ring.
2. Description of the Related Art
In a cam mechanism including a cam ring in which a linearly guided movable member (linearly movable member) is provided with a set of cam followers while the cam ring is provided on a peripheral surface thereof with a corresponding set of cam grooves in which the set of cam followers are respectively engaged so that the linearly movable member moves forward or rearward along the rotational axis of the cam ring by a rotation of the cam ring, the maximum amount of movement of the linearly movable member in the rotational axis direction of the cam ring is generally smaller than the axial length of the cam ring. For instance, in the case where the opposite ends of each cam groove are formed as closed ends relative to the front and rear ends of the cam ring, respectively, the cam ring needs to have a sufficient axial length for covering not only a cam-groove forming portion of the cam ring on which the set of cam grooves are formed but also front and rear end closing portions (front and rear end walls) in the rotational axis direction of the cam ring which close the opposite ends (front and rear ends) of each cam groove, respectively. These front and rear end closing portions cannot be used for moving the linearly movable member because no part of the set of cam grooves is formed on either the front end closing portion or the rear end closing portion. Strictly speaking, the range of movement of each cam follower in the rotational axis direction of the cam ring corresponds to the length of the cam-groove forming portion in the rotational axis direction of the cam ring from which the diameter of the cam follower is subtracted. Therefore, if the thickness of the front end closing portion, the thickness of the rear end closing portion, the diameter of each cam follower and the effective length of the cam ring in the rotational axis direction are represented by A, B, C and D, respectively, the maximum amount of movement E of the linearly movable member in the rotational axis direction of the cam ring is represented by the following equation:E=D−A−B−C. 
To provide each cam groove with an opening, either end of the cam groove is extended to be formed as an open end which penetrates through either the front end closing portion or the rear end closing portion. However, this extended portion of the cam groove is used only when the cam mechanism is assembled or disassembled and accordingly does not serve as operating portion for controlling movement of the linearly movable member when it is in operation. Accordingly, even if one end of each cam groove is formed as an open end through which the associated cam follower is inserted into the cam groove, the maximum amount of movement of the set of cam followers is substantially represented by the aforementioned equation.
The above described cam mechanism is often incorporated in a photographing lens. In photographing lenses produced in recent years, especially zoom lenses having a high zoom ratio, it has been required to increase the range of movement of one or more movable members such as a lens frame in the optical axis direction on one hand; on the other hand, there has been a heavy demand for the miniaturization of the lens barrel. For instance, a simple way to increase the aforementioned maximum amount of movement E of the linearly movable member and the like in the rotational axis direction of the cam ring (i.e., the optical axis direction) is to increase the effective length D of the cam ring in the rotational axis direction. However, this runs counter to the miniaturization of the lens barrel.